Presentation of foreign antigens by MHC class II gene products is a central arm of immunity. The process of MHC class II-dependent antigen presentation involves several discrete steps mediated by proteolytic enzymes: generation of peptides for their subsequent presentation as antigens in peptide complexes with MHC alpha/beta dimers and the stepwise breakdown of the class II-associated molecular chaperone, the invariant chain (Ii). Degradation of Ii promotes loading of MHC class II alpha/beta with newly formed peptides. A single protease, cathepsin S, was recently found to be essential to the process of Ii breakdown and efficient MHC class II peptide loading. These studies are aimed at elucidating the mechanisms by which cathepsin S promotes antigen presentation and at understanding the consequences of inhibition of cathepsin S on MHC class II function. Toward that end, cellular models of antigen presentation in which the activity of cathepsin S, and other cysteine proteases can be manipulated have been developed. Ii resistant to cleavage of cathepsin S will be generated by site-directed mutagenesis and the function of mutant Ii assessed in vitro and in vivo. The importance of cathepsin S in vivo to MHC class II-dependent immune responses, including allergic pulmonary reactions, will be tested by creating a selective, systemic deficiency of cathepsin S with low molecular weight protease inhibitors. And finally, mice with targeted disruption ("knockout") of the cathepsin S gene are being generated in order to assess the consequences of long-standing cathepsin S deficiency on MHC class II function and to provide a fertile background on which to search for additional proteases important to antigen presentation. These studies should provide new information on the basic events important to MHC class II peptide loading and establish whether there is a clear rationale for therapeutic inhibition of cathepsin S to suppress inflammation in lung diseases promoted by MHC class II-dependent immune reactions such as asthma, hypersensitivity pneumonitis, and transplant rejection.